Totally water-soluble N-doped Carbon dots (N-CDs) were synthesized by a green hydrothermal method from biomass using Highland barley as a carbon source and ethanediamine as nitrogen source. TEM and XRD showed the graphitic amorphous structure and narrow diameter distribution of these N-CDs. N-doping to the crystal lattice and carrying many hydrophilic groups on the surface of N-CDs were verified by XPS and FT-IR. The as-synthesized N-CDs emitted strong blue fluorescence at 480 nm and owned a relatively high quantum yield of 14.4%. The product also could sensitively and selectively detect Hg²⁺ ions in the range of 10–160 μM and the limit of detection was equal to 0.48 μM.

This work reports on a novel fluorescent sensor 1 for Cd²⁺ ion based on the fluorophore of tetramethyl substituted bis(difluoroboron)-1,2-bis[(1H-pyrrol-2-yl)methylene]hydrazine (Me₄BOPHY), which is modified with an electron donor moiety of N,N-bis(pyridin-2-ylmethyl)benzenamine. Sensor 1 has absorption and emission in visible region, at 550 nm and 675 nm, respectively. The long wavelength spectral response makes it easier to fabricate the fluorescence detector. The sensor mechanism is based on the tunable internal charge transfer (ICT) transition of molecule 1. Binding of Cd²⁺ ion quenches the ICT transition, but turns on the π − π transition of the fluorophore, thus enabling ratiometric fluorescence sensing. The limit of detection (LOD) was projected down to 0.77 ppb, which is far below the safety value (3 ppb) set for drinking water by World Health Organization. The sensor also demonstrates a high selectivity towards Cd²⁺ in comparison to other interferent metal ions.

A novel fluorescent Probe 1, based on phenanthro-imidazole has been developed as an efficient chemosensor for the trace detection of copper ions (Cu²⁺). Probe 1 demonstrated sensitive fluorescence quenching upon binding with Cu²⁺ through 1:1 stoichiometric chelation. The detection limit for Cu²⁺ ions was projected through linear quenching fitting to be as low as 2.77 × 10⁻⁸ M (or 1.77 ppb). The sensing response was highly selective towards Cu²⁺ with minimal influence from other common metal ions, facilitating the practical application of Probe 1 in trace detection.